Valve guidance for an exhaust gas valve of an internal combustion engine

The present invention relates to a valve guidance for a disk valve of an internal combustion engine used for the control of an exhaust gas channel which includes a guide body with good heat-conducting and sliding properties for the valve stem longitudinally guided within the same.

Very high demands are made of such a valve guidance for a disk valve which serves for the control of the hot exhaust gases of an internal combustion engine. In order to keep small the friction coefficient and therewith the mechanical power loss, a material with good sliding qualities, for example, a bronze or brass alloy must be used. A wear then occurs to a lesser extent at the disk valve made of steel rather than at the more soft valve guidance, especially at the inlet place of the valve stem in the valve guidance. For at this place the oil carbon which becomes stuck at the disk valve penetrates into the valve guidance and causes a very strong wear.

Ceramic bushings are proposed in the DE-OS No. 33 18 899 which are quite wear resistant but do not possess very good slide properties. Additionally, the heat-conducting ability of ceramic material is not particularly good so that the heat cannot be conducted sufficiently rapidly from the valve stem of the disk valve to the cylinder head, respectively, to another housing in which the valve guidance is seated.

The object of the present invention resides in providing a wear-resistant valve guidance with good heat conducting and sliding properties.

The underlying problems are solved according to the present invention in that the guide body includes in the area projecting into the exhaust gas channel a hard metal coating. If the valve stem is longitudinally guided in a guide body which, on the one hand, has good sliding properties and is a good heat conductor and, on the other hand, has a hard metal coating at the inlet place of the valve stem, the requirements for a good heat transfer and wear-resistant construction of the valve guidance are optimally fulfilled. The hard metal coating can be applied either on the guide body directly by plasma-spray process, or can be pressed into the guide body as hard metal bushing. Alternatively thereto, a guide bushing may be used as guide body which is inserted into the cylinder head of the internal combustion engine and whose end projecting into the exhaust gas channel carries the hard metal coating. Also in this case, the hard metal coating can be constructed as layer applied by plasma-spray process or can be pressed over the end face of the guide bushing as pot-shaped sleeve. In all cases, the wear resistant hard metal coating scrapes off from the valve stem the oil carbon adhering thereto and thus prevents the penetration thereof into the gap between valve stem and guide body. At the same time, the hard metal coating serves as corrosion protection against the chemically aggressive exhaust gases. Hard metal in the sense of the present invention is an alloy of iron-like components which also includes high alloyed high-grade steels.

These and other objects, features and advantages of the present invention will become more apparent from the following description when taken in connection with the accompanying drawing which shows, for purposes of illustration only, several embodiments in accordance with the present invention, and wherein:

FIG. 1 is a partial cross-sectional view through a cylinder head with a valve guidance in accordance with the present invention;

FIG. 2 is a partial cross-sectional view through a cylinder head with a valve guidance having a hard metal coating in accordance with the present invention; and

FIG. 3 is a partial cross-sectional view through a valve guidance with a hard metal sleeve in accordance with the present invention.

Referring now to the drawing wherein like reference numerals are used throughout the various views to designate like parts, a cylinder head 1 of an internal combustion engine includes an exhaust gas channel 2 which is controllable by a disk valve 3 which in the closing position abuts at a seating ring 4. According to FIG. 1, the valve stem 5 of the disk valve 3 is longitudinally guided in a guide bore of the cylinder head 1 cast of light metal, whereby the cylinder head 1 serves directly as guide body and conducts the heat away from the valve stem 5. A hard metal bushing 7 is pressed into an eye 6 protruding into the exhaust gas channel 2, the inner diameter of the bushing 7 is approximately equal to the diameter of the guide bore. When the disk valve 3 is displaced from its lower open position upwardly into the closing position, the hard metal bushing 7 abutting at the valve stem 5 acts, so to speak of, as scraper and keeps the oil carbon adhering at the valve stem 5 away from the guide bore.

According to FIG. 2, a guide sleeve 8 of a copper alloy, for example, of brass, is pressed into a bore of the cylinder head 1, which produces a good slide guidance for the disk valve 3 made of steel and at the same time has a good heat-conducting coefficient. Over the end protruding into the exhaust gas channel 2, a hard metal coating 9 is applied onto the guide bushing 8 by plasma-spray process, which covers the end face of the guide bushing 8 and the outer cylindrical area adjoining the same.

After the spraying-on process, the hard metal coating 9 is ground to finished dimension.

The end face coating acts like the hard metal bushing 7 of FIG. 1 as scraper for the oil carbon and assures that the same cannot penetrate into the guide sleeve 8. The coating of the guide sleeve 8 along the outside thereof protects the same against corrosive attack of the hot exhaust gases.

In the alternative, according to FIG. 3, a pot-shaped hard metal sleeve 10 is pressed over the guide sleeve 8 at the end protruding into the exhaust gas channel 2. The wall thickness of the hard metal sleeve 10 amounts to about 1.5 mm. and is slightly greater than the layer thickness of a high grade steel coating 9 applied by plasma process, whose layer thickness amounts to about 0.2 to 0.8 mm.

In a preferred embodiment, an alloy of 40% cobalt, 22.5% nickel, 25.5% chromium and 12.2% of tungsten by weight is chosen for the hard metal coating 9, respectively, the hard metal bushing 7 which is commercially available under the designation "Stellit."

Such a hard metal or high-grade steel coating can also be utilized for the valve guidance of an exhaust gas valve which is used in a turbocharger installation, for example, as by-pass valve. It also renders in that case the valve guidance so wear-resistant that no noticeable wear is indicated even after an operating period of several thousand hours. As the hard metal coating abuts at the valve stem 5 only over a very short distance, the heat conduction from the valve stem 5 is not significantly impaired.

While we have shown and described several embodiments in accordance with the present invention, it is understood that the present invention is not limited thereto but is susceptible of numerous changes and modifications as known to those skilled in the art. For example, the alloy composition can be varied as known to those skilled in the art. Thus, the present invention is not limited to the particular preferred alloy indicated hereinabove, but any other equivalent alloy may be used. Consequently, we do not wish to be limited to the details shown and described herein but intend to cover all such changes and modifications as are encompassed by the scope of the appended claims.